# MKV to MP4: remux your recordings in seconds

For OBS recordings and other Matroska files. H.264 and AAC are copied as they are; anything else is re-encoded. Free, no sign-up.

Your file never leaves your device: the tool runs inside your browser. Up to 1 GB. Use it at https://opusmotion.com/tools/mkv-to-mp4.

## How to use it

1. Drop the MKV, or tap the box to pick it.
2. H.264 and AAC are remuxed into the MP4 in seconds. Other codecs are re-encoded to H.264 and AAC, on your device.
3. Download the MP4, or put the clip in a launch template.

The first time, your browser downloads the video engine (31 MB, once); after that it opens from the cache.

## About MKV files

MKV (Matroska) is an open container that can hold almost any codec, several audio tracks and subtitles in one file.

OBS Studio can record to MKV, and many recordings are MKV for a good reason: an MKV stays playable if OBS or the computer crashes in the middle of a recording, while a regular MP4 that wasn't finished can't be opened.

Most MKV recordings hold H.264 video and AAC sound, which an MP4 takes as they are. Moving them over without re-encoding is what OBS calls remuxing: it takes seconds and changes nothing in the picture.

An MP4 made here keeps the first video track and the first audio track. Subtitles and extra audio tracks in the MKV aren't carried over.

## Questions

### Why does OBS record in MKV?

Because an MKV survives a crash. If OBS or the computer stops in the middle of a recording, the MKV still plays up to that point, while an MP4 that wasn't finished can't be opened. That's why it's common to record to MKV and remux to MP4 afterwards; OBS has a Remux Recordings command for it.

### Is this the same as OBS's Remux Recordings?

For H.264 and AAC recordings it's the same copy, without opening OBS and on any device, a phone included. When the MKV holds other codecs, this tool re-encodes them instead of stopping.

### What happens to subtitles and extra audio tracks?

The MP4 keeps the first video and the first audio track. Subtitles and any other audio tracks stay in the MKV.

### Why won't my MKV upload or play?

Many apps and sites (social networks, slide decks, video editors) don't take MKV files. An MP4 with H.264 and AAC is the one they all accept.

### Is my video uploaded anywhere?

No. The conversion runs inside your browser, with ffmpeg compiled to WebAssembly. Your file is read where it is and never sent to a server. The first time, your browser downloads the video engine (31 MB, once); after that it opens from the cache.

### What's the largest file it takes?

Up to 1 GB when the video is copied. When it needs re-encoding: up to 500 MB and 2 minutes on a computer, or 200 MB and 1 minute on a phone or low-memory device. The limit is decided after reading the codecs. The MP4 is kept in your browser's memory until you download it, so a phone, which has less memory, may not manage a very large file: if one fails there, try it on a computer.

## More free tools

- [Video converter](https://opusmotion.com/tools/video-converter.md): Any video to MP4: MOV, WebM, MKV, AVI and more, copied without re-encoding when it can be
- [MOV to MP4](https://opusmotion.com/tools/mov-to-mp4.md): iPhone videos, Mac screen recordings and QuickTime files to MP4, usually in seconds
- [WebM to MP4](https://opusmotion.com/tools/webm-to-mp4.md): Browser recordings and web videos (VP8, VP9) to an MP4 that plays everywhere
- [All free tools](https://opusmotion.com/tools.md)